The present invention relates generally to optical systems, and more particularly to an optical unit that collimates a beam radiated from a light source and scans in a predetermined direction. The present invention lends itself, for example, to a laser optical system for an electrophotographic recording device, and is applicable not only to a discrete printer, but generally to a combination apparatus having a printing function such as a photocopier and a facsimile unit.
The "electrophotographic recording device" by which we mean is a recording device employing the Carlson process as described in U.S. Pat. No. 2,297,691, as typified by a laser printer, and denotes a nonimpact image-forming device that provides a recording by depositing a developer as a recording material on a recorded medium (e.g., printing paper, and OHP film). The laser optical system as an optical system for exposure is typically a unit that includes a variety of light sources (e.g., a semiconductor laser, a He--Ne gas laser, a Ar gas laser, a He--Cd gas laser), a collimating lens, a rotating mirror, an f-.theta. lens, etc., serving to form a desired latent image on a photosensitive drum.
A laser printer as an example of the electrophotographic recording device has characteristics such as an excellent operability and cost efficiency, high-quality image formation, and a reduced vibration and noise during printing, and is thus expected to be increasingly demanded in future along with recent prevailing office automations. The laser printer generally includes a photosensitive drum and an optical unit for forming a latent image on the photosensitive drum by exposure to light. The photosensitive drum is uniformly negatively charged by a pre-charger, and exposed by a laser beam emitted from the optical unit, whereby an area on which a developing agent (toner) is to be deposited is discharged to form a latent image. The latent image is thereafter visualized into a toner image by a development device, and the toner image is transferred onto a printing paper by a transfer unit. A fixing device fixes the toner image that has been transferred on the printing paper, and the printing paper is then ejected.
The optical unit typically includes a laser beam source that radiates a laser beam, a collimator section that collimates (or renders parallel) the laser beam, a polygon mirror (rotating mirror) that allows the laser beam to change its traveling direction to make a scan, an f-.theta. lens that corrects a distortion of the laser beam, a print start detector section that adjusts print timing, and other necessary mirrors. A semiconductor laser providing a low cost and excellent maintainability has been predominantly used for the laser beam source.
The optical unit using the semiconductor laser radiates, so that the longer the radiating distance is, the wider it spreadout from the light source as a vertex of cone (a laser beam with a spreading angle from a point light sourc)e. The collimator section, which is located near the light source, collimates the laser beam to make a parallel beam fit for the exposure to the photosensitive drum. In order to expose a larger area in a single operation and speed up the writing action onto the photosensitive drum, the number of semiconductor lasers to be provided is normally plural, i.e., two or more, and the equal number of collimator sections are provided accordingly.
The collimator section includes a cylindrical lens barrel and a collimating lens. The lens barrel is fitted with and holds the collimating lens, and intercepts extraneous light. The collimating lens, which serves to collimate a radiated beam, is stuck at a whole face or several spots of its circumference to the lens barrel typically with a resinous adhesive in view of its manufacturing cost. Incidentally, an injection of the adhesive is manually given with an injector. The collimated beam is reflected by a rapidly rotating polygon mirror, passes through the f-.theta. lens, and then scans the photosensitive drum for a desired area to be exposed. The laser light that has passed through the f-.theta. lens exposes the photosensitive drum, and forms a latent image under print timing by the print start detector section.
Increasing demand for high-quality image formation in recent years has required the collimating lens that determines a traveling direction of the laser beam to be accurately attached in the lens barrel to form a precise latent image on the photosensitive drum. In a conventional optical unit, however, thermal deformation of the adhesive for the collimating lens would disadvantageously displace the collimating lens in the lens barrel, so as to deteriorate an image quality.
Associated with a use of the laser printer, the optical unit's temperature climbs up to about 60.degree. C. by heat generated in the fixing device, a motor, a printed board or the like in the printer, and a motor for the polygon mirror or a control printed board or the like in the unit. This would cause a thermal expansion of the adhesive applied onto a whole face or several spots of the circumference of the collimating lens. A variety of amounts, forms, and positions of the adhesive around the circumference of the lens leads to a variety of thermal deformations of an adhesive layer, and thus the collimating lens would be displaced in an unforeseeable direction, for example, by about 10-30 .mu.m, changing a laser-beam emitting position.
The shift direction of the collimating lens may be represented by a main scanning direction and a sub-scanning direction, but acceptable shift amount in the sub-scanning direction is much smaller than the acceptable shift amount in the main scanning direction. The main scanning and sub-scanning directions respectively correspond to the polygon mirror's circumferential and height directions. Even if the laser beam is displaced in the main scanning direction, the mirrors and lenses in them subsequent stage may generally detect the beam using the rotating polygon mirror. Therefore, based on their detection, the displaced beam may be adjusted by changing timing of the laser beam emission as disclosed in Japanese Laid-Open Patent Application No. 9-76559 or by shifting the beam using a refractive index of glass as disclosed in Japanese Laid-Open Patent Application No. 10-260368. However, the beam deviated in the sub-scanning direction would move in the height direction so that the polygon mirror cannot lead the beam to fall on the subsequent mirrors and lenses, and the subsequent mirrors and lenses thus cannot detect the beam. Consequently, that would disadvantageously make its latent image formation impossible or imperfect, or its print timing unable to be properly detected, thereby disabling printing function.
Further, in the optical unit equipped with two semiconductor lasers, the displacement of the collimating lenses would disadvantageously change its beam width, thereby making an contour of the latent image thick, or otherwise, so that a high-quality image could not be obtained.
To prevent the displacement of the collimating lens in the sub-scanning direction at high temperature, it is conceivable to provide a cooling device for cooling a lens and its surroundings or an adjustment means for detecting and correcting the displacement of the collimating lens in the sub-scanning direction, but this would unfavorably increase the device's complexity and manufacturing cost.